This invention relates to a process for the preparation of zero valent bis-arene transition metal compounds. More particularly, the present invention comprises the reduction of a bis-arene transition metal (I) compound with a highly-electropositive metal of Group IA or IIA of the Periodic Table of Elements in a substantially anhydrous ether solvent.
Zero valent bis-arene transition metal compounds are known in the art. Many of these compounds, and methods for their preparation, are disclosed in the comprehensive study of bis-arene transition metal complexes by W. E. Silverthorn, Adv. Orgmet. Chem., 13, pp. 47-137 (1975) and the references cited therein. The Silverthorn article and references therein are incorporated herein by reference. It is generally recognized that a zero valent bis-arene transition metal compound has the following structural formula: EQU [(Arene).sub.2 M].degree.
wherein M represents a transition metal and arene represents an aromatic hydrocarbon containing at least one six-membered ring pi-bonded to said transition metal.
.alpha.-Olefin polymerization catalysts containing zero valent bis-arene transition metal compound are also known in the art. U.S. Pat. No. 3,123,571 (Darrell W. Walker et al.) and U.S. Pat. No. 3,157,712 (Darrell W. Walker, et al.), incorporated herein by reference, describe the use of a number of zero valent bis-arene transition metal compounds in catalyst compositions for polymerization of olefins. Polymerization of ethylene using zero valent bis-benzenechromium in the presence of oxygen is described by H. Yamazaki et al., Mem. Inst. Sci. and Ind. Res., Osaka Univ., 20, p. 107 (1968). This article is also incorporated herein by reference.
Various methods for the preparation of zero valent bis-arene metal compounds are described in the aforementioned Silverthorn article and references therein. Two appear to have gained wider application than the other methods.
The first method is a two-step wet chemical synthesis comprising (1) formation of bis-arene transition metal (I) compound wherein the metal is in 1+ valence state by reacting transition metal halide, an arene, aluminum trihalide and aluminum metal, and (2) hydrolyzing the reaction product to yield zero valent bis-arene transition metal compound. The hydrolysis step involves disproportionation of the bis-arene transition metal (I) compound to the desired zero valent bis-arene transition metal compound, and transition metal compound wherein the transition metal is in 2+ or 3+ valence state, depending on the pH of the solution. Disproportionation of the bis-arene transition metal (I) compound can be represented by the following reactions: ##STR1## wherein M represents a transition metal.
Although this method is widely used, its disadvantages are that it involves many manipulations of very air-sensitive compounds and the maximum theoretical yield of zero valent bis-arene transition metal compound is about 67%, based on the transition metal, due to the disproportionation. Thus, about one-third of the transition metal is not utilized for the production of zero valent bis-arene transition metal compound in this method.
Preparation of various zero valent bis-arene transition metal compounds by this method is described in the aforementioned Silverthorn study, and references therein, and particularly in F. Calderazzo, Inorg. Chem., 3, 810(1964); E. O. Fischer and J. Seeholzer, Z. Anorg. Allgem. Chem., 312, 244(1961); E. O. Fisher and A. Rechziegel; Chem. Ber., 94, 2204(1961); F. Hein and K. Kortte, Z. Anorg. Allgem. Chem., 307, 52(1960); E. O. Fischer et al., Chem. Ber., 93, 2065(1960); and E. O. Fischer and H. Kogler, Chem. Ber., 90, 250(1957).
The second method is metal vapor synthesis which involves direct condensation of transition metal atoms are arene at liquid nitrogen temperature to produce zero valent bis-arene transition metal compound. Compounds, such as zero valent bis-benzene titanium and zero valent bis-(hexafluorobenzene)chromium, which were not accessible by the aforementioned two-step wet chemical synthesis, have been prepared by this method. However, this method is unsuitable for the preparation of bis-arene compounds of transition metals of the second- and third-row of the Periodic Table, because of much lower volatility of these metals. Accordingly, there is a need for an improved process for the preparation of zero valent bis-arene transition metal compounds.
Preparation of various zero valent bis-arene transition metal compounds by metal vapor synthesis is also described in the aforementioned Silverthorn study and references therein, and particularly in F. W. S. Benfield et al., J. Chem. Soc. Chem. Commun., 866(1973); R. Middleton, J. Chem. Soc., Dalton Trans. 120(1973); and P. S. Skell, J. Amer. Chem. Soc., 95, 3337(1973).
The general object of this invention is to provide an improved synthesis of zero valent bis-arene transition metal compounds. Another object is to provide an improved yield of the desired zero valent bis-arene transition metal compounds. Other objects appear hereinafter.